Charge Coupled Device (CCD) imaging arrays have made possible high quality imagers now used in consumer camcorder equipment, scanners for FAX machines, and video cameras for a wide range of applications including video-conferencing, and portable equipment for professional TV broadcasting.
With the advent of multimedia communications, there arises a need for low cost solid state image sensors to complement computers and communication devices and thus realize practical video telephones and the like. An image input device is central to any teleconferencing and multimedia application. Recently, CMOS image sensors have been recognized as a viable candidate for the image input device. CMOS image sensors also have utility in other fields such as robotics, machine vision, security surveillance, automotive applications and personal ID systems through fingerprint/retina scan. A distinct advantage of CMOS image sensors (or imagers) is that signal processing circuits can be readily integrated on the same chip as the image, thus enabling design of smart, single-chip image acquisition systems. CMOS imagers can be manufactured at lower cost than that of conventional charge coupled devices (CCDs) using conventional, preinstalled CMOS fabrication lines without any process modification.
Since portable electronic equipment operates using batteries, it is preferable to design such equipment to provide low-power consumption. The use of a low-power image device enables portable electronic equipment to consume less power.
As is well known, light is analog data that varies continuously. For discrete signal processing, analog data is converted into digital data. CMOS image devices (or imagers) incorporate a device that detects the light as an analog signal and converts a detected analog signal into digital data. For this, CMOS image devices typically incorporate an analog-to-digital converter. In this respect, one approach to realize a low-power image device is to reduce power consumption of analog-to-digital converters incorporated in the image device. Accordingly, there is a need for an analog-to-digital converter capable of reducing power consumption, which can be used for a CMOS image device.